In exhaust gas exhausted from combustion engines such as lean-burn type gasoline engines, diesel engines, various harmful substances derived from fuel and combustion air are contained depending on their structures and types. These harmful substances include hydrocarbons (HC), soluble organic fraction (also referred to as SOF), soot, carbon monoxide (CO), nitrogen oxides (NOx), and the like, and these substances are regulated by the Clean Air Act. And as a purification method for these substances, a contact treatment process in which exhaust gas is purified by contacting with a catalyst has been practically used.
In addition, in such a lean combustion engine, combustion temperature is controlled by such a procedure that an optimum amount of air for combustion is supplied corresponding to a type and feed rate of fuel, to inhibit amounts of harmful substances to be generated. However, it is not that, in all type of combustion engines, air and fuel can be always controlled in an ideal state, and large amounts of harmful substances such as nitrogen oxides are sometimes generated due to incomplete combustion. Such circumstances are same in internal combustion engines, and in the case of diesel engines, nitrogen oxides tend to be exhausted because it has a structure in which the engine is operated by lean combustion. Among them, in the case of a diesel engine that is mounted in an automobile, since its operation conditions always vary, it had been particularly difficult to properly inhibit generation of harmful substances.
Among the harmful substances exhausted in such way, as a means to purify (denitrate) NOx, a technology has been studied in which NOx is contacted with a catalyst containing as main components titanium oxide, vanadium oxide, zeolite, and the like under the presence of an ammonia (NH3) component, to be reduced and denitrated. The catalyst to be used for this purpose is known as Selective Catalytic Reduction Catalyst (hereinafter, also referred to as SCR).
In this SCR catalyst using this NH3 component as a reducing agent, NOx is finally reduced to N2 mainly by the reaction equations (1) to (3) shown below.4NO+4NH3+O2→4N2+6H2O  (1)2NO2+4NH3+O2→>3N2+6H2O  (2)NO+NO2+2NH3→2N2+3H2O  (3)
In a denitration catalyst system utilizing such reaction mechanisms, gasified NH3 may be used as a reducing component, but NH3 itself has harmful effects such as an irritating odor, etc. Therefore, such a system has been proposed that an urea aqueous solution as a NH3 component is added in the upstream of denitration catalyst to generate NH3 by thermal decomposition or hydrolysis and exert denitration performance as a reducing agent by the reactions of the above equations.
The reactions to obtain NH3 by decomposition of urea in such way are represented by the following equations.NH2—CO—NH2→NH3+HCNO (thermal decomposition of urea)HCNO+H2O→NH3+CO2 (hydrolysis of isocyanic acid)NH2—CO—NH2+H2O→2NH3+CO2 (hydrolysis of urea)
In the purification of NO in exhaust gas, in the denitration reactions (1) to (3), molar ratio of NH3/NOx should be 1.0 in theory, but NH3 has sometimes to be supplied in a higher NH3/NOx ratio to obtain a sufficient purification performance for NOx, in the case of a transitional operation condition during operation of a diesel engine or in the case of inadequate space velocity or gas temperature. In such case, a risk had been pointed out that unreacted NH3 leaks out (hereinafter, also referred to as slip or NH3-slip) and causes a secondary pollution such as environmental pollution.
On the basis of these problems, various catalyst technologies had been studied as SCR (see: JP-A-2004-524962). In addition, in order to purify NH3 slipped from SCR, such a process had been studied that the slipped NH3 is purified by oxidation as shown by the following reaction equation (4) by placing a NH3-purification catalyst in which platinum (Pt), palladium (Pd), rhodium (Rh), or the like is supported on a base material of alumina or the like, in the subsequent stage of SCR.2NH3+3/2O2→N2+3H2O  (4)
However, since the above-described catalyst to purify NH3 uses a noble metal component having a high oxidizing ability such as platinum, palladium, rhodium as a catalytically active species, there was a problem that the catalyst could cause the new generation of NOx component such as N2O, NO, NO2 in addition to the oxidation of NH3, as shown by the following reaction equations (5) to (7).2NH3+5/2O2→2NO+3H2O  (5)2NH3+7/2O2→2NO2+3H2O  (6)2NH3+2O2→N2O+3H2O  (7)
In order to inhibit such generation of NOx, a purification catalyst having a slipped NH3 measure has been proposed, in which a component having an oxidative decomposition activity for NH3 in a lower layer and one or more types of oxides selected from titanium, tungsten, molybdenum and vanadium which do not have an oxidative decomposition activity for NH3 in an upper layer are laminated as a denitratively active component (see: JP-A-10-5591). According to JP-A-10-5591, a thin catalyst layer having only a denitration activity has been formed on a surface layer, where firstly a denitration reaction proceeds and most of NO and NH3 are utilized in the denitration reaction. Thereafter, remaining NH3 and NO diffuse to an inner part of the catalyst, and reach to an inside layer of the catalyst (inner layer) having a denitration activity and an oxidative activity. In this case, since NH3 is present in large a excess against NO in the inner layer, not only oxidative decomposition of NH3 but also denitration reaction can be promoted. It has been reported that only a final surplus NH3 is oxidatively decomposed by a noble metal-supported component in the inner layer, and an additional generation of NOx can be inhibited by purification of slipped NH3. The catalyst in JP-A-10-5591 uses vanadium as an essential component which has a high reaction efficiency, and has been thought to be a useful denitration component. However, vanadium itself is a harmful heavy metal, and its use is not desirable because of a fear that it could vaporize into exhaust gas when used as a catalyst, therefore, some automobile catalyst manufacturers avoid using it.
In addition, a process has been studied in which a catalyst for purifying NH3 slipped from SCR in the subsequent stage of a selective catalytic reduction catalyst (SCR) is placed in order to purify slipped NH3 (see: JP-A-2005-238195 and JP-A-2002-502927). Although purification of slipped NH3 proceeds by arranging a catalyst to oxidize NH3 in the subsequent stage in such a way, the process is accompanied by a risk that additional NOx is generated due to a presence of a highly active noble metal catalyst on the surface of the carrier, and cannot be regarded as satisfying one against the regulation for NOx which is becoming increasingly severe in recent years.
In addition, for such problem, in order to prevent the generation of additional NO in purification of slip NH3 without using vanadium as a catalyst component, it is considered to increase a volume of SCR by which NH3 is consumed. Although catalytic reaction is promoted in proportion to the surface area of a catalyst, the volume of catalyst to be loaded and its placement are limited in the case of automobile use, therefore, the measure to increase simply a volume of catalyst cannot be said to be a practical solution.
In addition, a technology has been proposed in which a combustible particle component such as soluble organic fraction, soot in exhaust gas is purified together with purification of NOx (see: JP-A-2002-502927). The combustible particle component is filtered out from exhaust gas using a filter. The filtered out combustible particle component deposits on the filter, and leads to the clogging of the filter, if the component is kept as it is. Therefore, the combustible particle component deposited on the filter is removed by combusting. For such combustion, oxygen and NO2 in exhaust gas are utilized. In the case of using NO2, combustion of the combustible particle component can proceed even at such a low temperature as of exhaust gas from diesel engines (see: JP No. 3012249).
However, since the combustion of combustible particle component is oxidation reaction even when NO2 is used, and a large amount of NO is contained in the exhaust gas exhausted after the combustion, some purification means for NOx is required in this method also. Therefore, purification of NOx is carried out in the subsequent stage of the combustion of combustible particle component by a filter by arranging SCR in the subsequent stage of the filter.
As described above, in the conventional NH3—SCR technologies, there is no catalyst which can inhibit generation of additional NOx and simultaneously provides a sufficient NOx purification performance and a slip NH3 purification performance, and a catalyst which has these performances all together has been earnestly desired.
[Patent Literature 1]: JP-A-2004-524962 (claim 10);
[Patent Literature 2]: JP-A-10-5591 (claim 1, claims 17,[0022] and [0023]);
[Patent Literature 3]: JP-A-2005-238195 (claims 1, [0009] and [0061]);
[Patent Literature 4]: JP-A-2002-502927 (claims 1 and [0013]);
[Patent Literature 5]: JP No. 3012249.